1. Field of the Invention
This invention relates to a semiconductor laser device and a manufacturing method therefor, and particularly to the control of reflectance of an end surface of a semiconductor laser device.
2. Description of the Related Art
Ordinarily, a semiconductor laser which is indispensable as a light source for an optical communication, an optical information processing and an optical measurement has a double hetero-structure in which an active layer (light emission layer) is sandwiched between an n-clad layer and p-clad layer at both sides thereof.
FIG. 1 is a perspective view of a semiconductor laser device. As shown in FIG. 1, when a forward voltage is applied to an active layer 4 composed of a semiconductor having small band gap, and an n-clad layer 3 and a p-clad layer 5 which are composed of semiconductor having large band gap, electrons and holes flow into the active layer 4 from the n-clad layer 4 and the p-clad layer 5, respectively. These carriers are trapped in the active layer 4 by an energy barrier due to a band gap difference at the heterojunction. This trapping of the carriers promotes effective recombination between the electrons and the holes, and produces spontaneously-emitting light. The spontaneous emission light further promotes subsequent recombination between the electrons and the holes. On the other hand, the end surface of the active layer 4 serves as a reflection mirror for an optical resonator, so that induced emission and light amplification are promoted while the light goes and returns in the optical resonator. In this case, by increasing injection current amount to some degree, laser oscillation is finally induced, and the output intensity of the light is rapidly increased. As a result, a laser beam having directivity and narrow spectral band width is emitted from an emission point 8.
An n-electrode 1 and a p-electrode 7 are used to control the injection current, and a cap layer 6 is used to reduce resistance for the p-electrode 7.
Semiconductor containing Al such as AlGaAs group, AlInP group or the like has been frequently used for the n-clad layer 3, the active layer 4 and the p-clad layer 5 of the semiconductor laser. Therefore, as shown in FIG. 1B, an end surface protection film 10 is grown to prevent oxidation of Al.
On the other hand, the end surface 9 serves as a reflection mirror for laser oscillation, and an improvement in characters such as high output, low power consumption, etc. is realized by growing an optical thin film on the end surface protection film 10 and varying reflectance efficiency of the mirror surface (hereinafter referred to as "end surface reflectance").
However, when a semiconductor laser device is installed into an optical system, light beam emitted from the semiconductor laser device returns back from the optical system, and thus noises occur. Therefore, the following method has been conventionally adopted to prevent the affection of the return beam.
FIG. 2 is a diagram showing an optical thin film forming method of a first related art. After an optical thin film 22 is formed on the end surface protection film 20 of the semiconductor laser device, using a photolithography, a laser beam or the like is irradiated from a light source 23 to a portion adjacent to an emission point 21 as shown in FIG. 2A to form an optical thin film 22a and vary its thickness, whereby the optical thin film 22a having high reflectance is formed in the neighborhood of the emission point 21 and an optical thin film 22b having low reflectance is formed at the other regions.
FIG. 3 is a diagram showing an optical thin film forming method of a second related art. As shown in FIG. 3A or 3B, a partition plate 50 is disposed in front of an end surface protection film 40 of the semiconductor laser device, and metal which is supplied from a deposition source is shielded by this partition plate 50 to vary the thickness of optical thin films 41a and 41b and the thickness of optical thin films 41c and 41d, respectively, whereby the optical thin films 41a and 41c having high reflectance are formed at the emission point 38 and the optical thin films 41b and 41d having low reflectance are formed at the other regions. FIG. 3C is another view of FIG. 3A which is viewed from a deposition source direction.
FIG. 4 is a diagram showing an optical thin film forming method of a third related art. As shown in FIG. 4, the thickness t of the semiconductor laser device is reduced and an optical thin film 62 having high reflectance is formed over the end surface.
In the first related art as shown in FIG. 2, the optical thin films 22a and 22b having different reflectance are formed by irradiation of a laser beam or the like, however, this method has a problem in the point that a device for irradiating the laser beam or the like and its control operation are complicated. Further, in the second related art as shown in FIG. 3, the optical thin films 41a and 41b and the optical thin films 41c and 41d which have different reflectance are formed using the partition plate 40, however, this method also has a problem in the point that a device for controlling the partition plate 40 is complicated. Still further, in the third related art as shown in FIG. 4, a yield is reduced due to cracking occurring in a process where a semiconductor laser device is thinned in a wafer state. In addition, in any methods as shown in FIGS. 2 to 4, restriction is imposed on a distribution of reflectance, and thus the improvement in characters of the semiconductor laser device is limited.